PPS fibers, which are superior in heat resistance and chemical resistance, are used for various applications, one of which is nonwoven fabric. Specifically, a wet-laid nonwoven fabric and a so-called electrically insulating paper each being composed of PPS fibers and containing amorphous PPS fibers (undrawn PPS fibers) as a binder are disclosed (JP 7-189169 A, JP 2004-285536 A). A wet-laid nonwoven fabric is obtained by mixing amorphous PPS fibers at the time of paper making, drying, and then performing pressurized heat treatment to fuse fibers with the amorphous PPS fibers. However, amorphous PPS fibers are problematic in that they have a high degree of shrinkage on dry heating and are inferior in dimension stability, and therefore they shrink at a time of drying in a papermaking step, so that wrinkles, blisters, uneven drying, and the like are generated in wet-laid nonwoven fabrics and good wet-laid nonwoven fabrics are not obtained. PPS fibers which has a low degree of shrinkage on dry heating and superior in thermal dimension stability are disclosed (JP 3-104923 A, JP 2003-221731 A), but these were crystallized PPS fibers (drawn PPS fibers) and were hardly used as a binder. Electrically insulating paper to be used for a capacitor, a transformer, a cable, and the like is required to have high dielectric breakdown strength. However, high dielectric breakdown strength was not able to be achieved by the technologies disclosed in JP 7-189169 A and JP 2004-285536 A. That is, it is conceivable that mixing amorphous PPS fiber as much as possible and melting it to fill voids therewith are effective for achieving a high insulation, but there was a problem that because an amorphous PPS fiber was poor in thermal dimension stability, it was poor in papermaking property and therefore its incorporation ratio could not be increased.
It could therefore be helpful to provide a PPS fiber that is amorphous, low in degree of thermal shrinkage, and suitable for a binder for papermaking or the like, a method for producing the same, as well as a wet-laid nonwoven fabric containing the above-mentioned PPS fiber, and a method for producing a highly insulating wet-laid nonwoven fabric.